Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
1997-12-24
2001-03-27
Lee, Eddie C. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S064000, C257S075000
Reexamination Certificate
active
06207971
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film transistor suitable for use in an active matrix type display apparatus and a method of fabricating the same.
A liquid crystal display (LCD) of an active matrix type which uses thin film transistors (TFTs) has recently been getting attention as a high-quality display apparatus. Dot matrix type LCDs, which have a plurality of pixels arranged in a matrix form, are generally classified into a simple matrix system and an active matrix system.
The active matrix type LCD includes pixels, pixel drive elements (active elements) and signal storage elements (storage capacitors or added capacitors) and drives a liquid crystal in a quasi-static manner which permits each pixel to store data. Each pixel drive element serves as a switch which is switched on or off in response to a scan signal. When the pixel drive element is enabled, a data signal (display signal) is transmitted via that pixel drive element to the associated display electrode, such that the liquid crystal is driven by the data signal. When the pixel drive element is disabled, the data signal is stored in the form of a charge in the associated signal storage element. The liquid crystal is kept driven by the discharging of the charge until the pixel drive element is switched on again. Even though the drive time assigned to a single pixel decreases as the number of scan lines increases, the liquid crystal is sufficiently driven. This prevents the contrast from becoming lower.
Thin film transistors (TFTs) are generally used as pixel drive elements. A TFT has an active layer comprised of a thin semiconductor film formed on an insulator substrate. The semiconductor film preferably includes an amorphous silicon film or a polycrystalline silicon film.
A TFT having an active layer comprised of an amorphous silicon film is called an amorphous silicon TFT, while a TFT having an active layer comprised of a polycrystalline silicon film is called a polycrystalline silicon TFT. The polycrystalline silicon TFT has a greater field effect mobility and higher drive performance than the amorphous silicon TFT. Because of these advantages, the polycrystalline silicon TFT can be used as a logic circuit element as well as a pixel drive element. The use of polycrystalline silicon TFTS, therefore, allows not only the display screen, but also a peripheral drive circuit, located at the periphery of the display screen, to be integrally formed on the same substrate. That is, the display screen and peripheral drive circuit can be formed in the same step.
FIG. 1
is a schematic block diagram of a typical active matrix type LCD. The LCD includes a display panel
101
, a gate driver
103
, and a drain (data) driver
104
.
The display panel
101
has a plurality of scan lines (gate lines) G
1
, . . ., Gn, Gn+1, . . ., and Gm, a plurality of data lines (drain lines) D
1
, . . ., Dn, Dn+1, . . ., and Dm running perpendicular to the gate lines G
1
to Gm, and a plurality of pixels
102
provided at the intersections of the gate lines G
1
to Gm and the drain lines D
1
to Dm. The gate driver
103
, connected to the gate lines G
1
to Gm, applies a gate signal (scan signal) to the gate lines G
1
to Gm. The drain driver
104
, connected to the drain lines D
1
to Dm, applies a data signal (video signal) the drain lines D
1
to Dm. Both drivers
103
and
104
form a peripheral drive circuit
105
. Either one of the drivers
103
and
104
or both are preferably formed on the same substrate on which the display panel
101
is formed. The LCD is generally called a driver-integrated (driver-incorporated) LCD. The gate driver
103
or the drain driver
104
may be provided on both sides of the display panel
101
.
FIG. 2
shows an equivalent circuit of each pixel
102
. The pixel
102
includes a liquid crystal (LC) cell LC having a display electrode (pixel electrode) and a common electrode. The LC cell LC is connected to both a TFT
106
and a supplemental capacitor C
S
which has a storage electrode and an opposing electrode. The TFT
106
has a gate connected to the gate line Gn, a drain connected to the drain line Dn, and a source connected to the display electrode of the LC cell LC and the storage electrode of the supplemental capacitor C
S
. The LC cell LC and the supplemental capacitor C
S
form a signal storage element. A voltage V
com
is applied to the common electrode of the LC cell LC. A predetermined voltage signal V
R
is applied to the opposing electrode of the supplemental capacitor C
S
. The common electrode of the LC cell LC is common to all the pixels
102
. The LC cell LC has a capacitor formed between the display electrode and the common electrode.
The writing characteristic and holding characteristic of the pixel
102
are important in improving the image quality. The writing characteristic shows how much the LC cell LC and the supplemental capacitor C
S
can write desired video signals per unit time based on the specifications of the display panel
101
. The holding characteristic shows how long the written video signals can be held. The supplemental capacitor C
S
is provided to increase the capacitance of the pixel to improve the holding characteristic. In other words, the supplemental capacitor C
S
supplements the LC cell LC with the capacitance.
When a positive voltage is applied to the gate of the TFT
106
via the gate line Gn, the TFT
106
is turned on and a data signal is applied to the drain line Dn. As a result, the capacitor of the LC cell LC and the supplemental capacitor C
S
are charged. If a negative voltage is applied to the gate of the TFT
106
, the TFT
106
is turned off. At this time, the capacitor of the LC cell LC and the supplemental capacitor C
S
hold the applied voltage. In other words, the pixel
102
holds a data signal as the data signal is applied to the associated one of the drain lines D
1
to Dm by controlling the voltage on the associated one of the gate line to G
1
to Gm. An image is displayed on the display panel
101
in accordance with the held data signal.
FIG. 3
is a cross-sectional view of a part of the conventional LCD display panel
101
which has polycrystalline silicon TFTs
106
of a bottom gate structure. It is preferable that the display panel
101
is of a transparent type.
A polycrystalline silicon film (active layer)
81
of the TFT
106
is formed as follows. First, an amorphous silicon film is formed on a gate insulator film
80
using CVD (Chemical Vapor Deposition). The gate insulator film
80
preferably includes a silicon nitride film
78
and a silicon oxide film
79
. Next, an excimer laser beam is irradiated on the surface of the amorphous silicon film to heat the amorphous silicon film, thus forming a polycrystalline silicon film. Laser annealing using an excimer laser beam is called ELA (Excimer Laser Anneal). The ELA scans with a line beam to anneal the entire surface of the amorphous silicon film.
It is preferable that chromium with a high thermal conductivity is used for the gate electrode,
76
, of the TFT
106
and the opposing electrode,
77
, of the supplemental capacitor C
S
. Therefore, the heat energy applied to a part of the amorphous silicon on the gate electrode
76
by the ELA is transmitted via the associated one of the gate lines G
1
to Gn, integral with the gate electrode
76
, and is diffused outside the irradiation area of the line beam. Consequently, the temperature of the part of the amorphous silicon film on the gate electrode
76
is lower than that of another portion of the amorphous silicon film. In other words, the energy provided to a part of the amorphous silicon film on the gate electrode
76
is lower than the energy given to another part of the amorphous silicon film above the insulator substrate
71
. It is preferable to set the laser energy to maximize the grain size of the polycrystalline silicon film
81
. When the laser energy exceeds the value that maximizes the grain size, the grain size becomes drastically smaller. Suppose that the laser energy has been set
Jinno Yushi
Minegishi Masahiro
Wakita Ken
Eckert II George C.
Lee Eddie C.
Ross P.C. Sheridan
Sanyo Electric Co,. Ltd.
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